Extreme ultraviolet (EUV) lithography, which is based upon exposure with the portion of the electromagnetic spectrum having a wavelength of 10-15 nanometers, can be used to print features with smaller critical dimension (CD) than other more conventional techniques, such as those utilizing deep ultraviolet (DUV) radiation. For example, an EUV scanner may use 4 imaging mirrors and a Numerical Aperture (NA) of 0.10 to achieve a CD of 50-70 nm with a depth of focus (DOF) of about 1.00 micrometer (um). Alternatively, an EUV scanner may use 6 imaging mirrors and a NA of 0.25 to print a CD of 20-30 nm although the DOF will be reduced to about 0.17 um.
Masking and reflection of EUV radiation brings about a unique set of challenges generally not encountered with DUV radiation. For example, a mask for DUV lithography is transmissive, and layers of materials such as chrome and quartz may be used to effectively mask or transmit, respectively, DUV radiation. Thus, a desired pattern on a DUV mask may be defined by selectively removing an opaque layer, such as chrome, to uncover portions of an underlying transparent substrate, such as quartz. However, virtually all condensed materials absorb at the EUV wavelength, so a mask for EUV lithography is reflective, and the desired pattern on an EUV mask is defined by selectively removing portions of an absorber layer (“EUV mask absorber”) to uncover portions of an underlying mirror coated on a substrate, the mirror, or reflective multilayer (“ML”), generally comprising a number of alternating layers of materials having dissimilar EUV reflectivity constants.
Selective removal of portions of the EUV mask absorber generally involves etching trenches through portions of the EUV mask absorber material, and the CD uniformity and bias are highly dependent upon the accuracy of such etching. Two particular issues are etch bias within etched trenches, and prevention of overetching to a depth greater than desired. These issues may be related, since material and technique pairings associated with anisotropic etching may be associated with overetching, while pairings associated with enhanced prevention of overetching may be associated with more isotropic etching patterns and negative trench etch bias. Given the geometric tolerances required during selective removal of EUV mask absorber portions for a preferred masking result, specialized materials, compositions, and processes are needed.